Micro-Embossing of Polymeric Substrates for Fluidic Self-Assembly

Fluidic Self-Assembly™ (FSA)™ has become a routine manufacturing process in the production of radio-frequency identification tags. FSA operates through the self-positioning of micro-devices into pre-prepared matching receptor sites in a substrate. Research at North Dakota State University has focused on extending the applications of FSA well-beyond the current production routine. This pursuit requires, among other modifications, substantive extrapolation of the size, depth, configuration, spacing and spatial density of receptor sites. Three different test wafer patterns (see Figure 5 for patterns having nominal sizes of 1050μ, 1500μ, μ2150 and 3050μ square receptors with different spacing between them) took into account the corner compensation structure dimensions, which are based on thickness of silicon mold wafer feature to be etched (see Figure 2). The embossing tool (silicon wafer) was patterned photo-lithographically and subsequently wet etched in a KOH 2:1 solution. Experiments suggest shorter tool life in the case of closely packed features (spacing ~ 0.5mm). Receptor profiles evaluated using both optical and mechanical inspection (see Figures 3 and 4) suggest that features having larger size (up to nominal size of 3050μ square) and thickness (nominal depths of 110μ and 210μ) can be embossed accurately for use in FSA by slightly increasing the embossing time in case of deeper receptors. It was also noticed that the relative receptor depths attained with respect to the thickness of the feature on the mold wafer was lower while embossing deeper receptor sites, leading to the conclusion that mold wafers must be etched longer in such cases. The embossed receptor sites were subsequently filled with micro-devices in accordance with the standard operating parameters of Fluidic Self-Assembly process. These sample experimental runs suggest receptors slightly deeper than the micro-devices facilitate higher yields (or fill rates) in FSA. However, in cases where the receptors are too deep relative to the micro-device (> 5μ), air-entrapment occurred between the micro-device and the bottom of the receptor site, which caused problems in post-FSA processes due to air expansion. This paper presents comprehensive guidelines for embossing larger and deeper receptors for effective use in FSA.